Bone Formation
Bone formation, i.e. osteogenesis, is essential for the maintenance of bone mass in the adult skeleton. It begins during prenatal development and persists throughout adulthood. There are two ways in which osteogenesis occurs: intramembranous ossification and endochondral ossification.
There are two types of cells that are important in osteogenesis. Osteoblasts, involved in both endochondral and intramembranous ossification, are the specialized cells in bone tissue that make matrix proteins resulting in the formation of new bone. These bone-forming cells are derived from mesenchymal osteoprogenitor cells. They form an osseous matrix in which they become enclosed as an osteocyte. They are capable of differentiating to other lineages such as adipocytes, chondrocytes and muscle (Bellows et al., 1994). Unlike osteoblasts, osteoclasts are used in endochondral ossification. They dissolve calcium previously stored away in bone and carry it to tissues whenever needed. Thus, while osteoblasts are associated with new bone growth, osteoclasts are associated with bone absorption and removal.
Bone is subject to constant breakdown and resynthesis in a complex process mediated by osteoblasts, which produce new bone, and osteoclasts, which destroy bones. In normal bone, the balance between osteoblast-mediated bone formation and osteoclast-mediated bone resorption is maintained through complex regulating interactions.
Conditions and Diseases Associated with Bone Formation
There are many deficiencies, diseases, and disorders associated with the skeletal system. Examples of a few include, but are not limited to, osteoporosis, bone cancer, arthritis, rickets, bone fracture, periodontal disease, bone segmental defects, osteolytic bone disease, primary and secondary hyperparathyroidism, Paget's disease, osteomalacia, hyperostosis, and osteopetrosis.
Accordingly, there is a need to develop methods of treating diseases associated with bone growth disorders, methods of hastening bone formation, methods of identifying agents that modulate (increase or decrease) bone formation, and methods of identifying genes associated with bone growth disorders.
ΔFosB
The AP-1 family of transcription factors consists of dimeric complexes of Fos-related (c-Fos, FosB, ΔFosB, Fra-1 and Fra-2) and Jun-related (cJun, JunB and JunD) proteins. These are basic leucine zipper proteins that modulate the transcription of a variety of genes via interactions with specific sequences on the promoters of target genes. Members of the AP-1 family participate in the regulation of bone cell proliferation and differentiation, and the promoters of several genes involved in bone formation contain AP-1 consensus sequences. In addition, several regulators of bone formation induce AP-1 expression in early osteoblast precursors (Grigoriades et al., 1993). Furthermore, mice overexpressing c-Fos develop osteosarcomas (Grigoriades et al., 1993, Wang et al., 1992) while c-fos knockout mice lack osteoclasts and develop osteopetrosis (Johnson et al., 1992; Wang et al., 1992). In contrast, no bone abnormalities have been described in fosB knockout mice (Gruda et al., 1996), nor in mice where FosB, Fra-2, c-Jun or JunB alone were overexpressed (Grigoriadis et al., 1993).
Despite numerous studies on the role of AP-1 family members in skeletal biology, the role of ΔFosB, a Fos-related protein that arises from alternative splicing of the fosB transcript, has never been analyzed. ΔFosB can be induced in a region-specific manner in brain in response to several types of chronic perturbation, including drugs of abuse, antipsychotic drugs, antidepressant drugs, seizures and lesions (Chen et al., 1998). Once induced, ΔFosB isoforms persist in brain for relatively long periods due to their extraordinary stability. Mice lacking the fosB gene show abnormal biochemical and behavioral responses to chronic administration of drugs of abuse or antidepressant treatments, consistent with an important role for ΔFosB in mediating long-term adaptations in the brain (Nestler et al., 1999).
The Relationship Between Adipogenesis and Osteogenesis
Adipocytes form an integral part of the stromal system of bone and marrow and participate in the establishment and maintenance of the hematopoietic microenviroment (Beresford et al, 1992). Adipocytic and osteogenic cells are the two main cell lines of the marrow stromal system. There is also evidence that they are derived from a multipotential stromal stem cell in the adult marrow (Beresford et al, 1992). Beresford et al. (1992) report that there is evidence for an inverse relationship between the expression of the adipocytic and osteogenic phenotypes in cultures of rat marrow stromal cells. Beresford et al. (1992) also report that this finding is consistent with the possibility that the regulation of adipogenesis and osteogenesis can occur at the level of a common precursor.
Gimble et al. (1995) describe the effects of bone morphogenetic proteins on adipogenesis in a multipotent murine bone marrow stromal cell line, BMS2 Gimble et al. (1995) report that bone morphogenetic proteins inhibit adipocyte differentiation in multipotent bone marrow stromal cells in vitro.
Diseases Associated with Adipogenesis
Diseases associated with adipogenesis include body weight disorders such as obesity, anorexia, cachexia, and nonshivering and shivering thermogenesis. Obesity is usually defined as body weight of more than 20% in excess of the ideal body weight. Obesity is associated with an increased risk for cardiovascular disease, diabetes, and an increased mortality rate (Grundy et al., 1990, Disease-a-Month 36:645-696). On the other hand, loss of appetite, diminished food intake, and loss of body weight are also problems associated with many diseases.
Thus, there is also a need to develop methods of treating diseases associated with adipogenesis, methods of identifying agents that modulate adipogenesis, and methods of identifying genes associated with adipogenesis.